1. Field of the Invention
The present invention relates to a short arc type discharge lamp which is used in the light source of a data projector and the like, and more particularly to an electrode structure of a short arc type mercury discharge lamp in which 0.15 mg/mm3 or more of mercury is sealed inside a discharge vessel of the lamp, or an electrode structure of a short arc type mercury discharge lamp in which at least an emission metal and halogen are sealed inside a discharge vessel of the lamp.
2. Description of the Related Art
In the light source of a projector such as a data projector and the like, in addition to high illuminance and high color rendering, miniaturization thereof are desired. In order to meet these demands, a short arc type superhigh-pressure mercury lamp that has 0.15 mg/mm3, or more, of mercury, or various emission metals and halogen sealed inside a discharge vessel of the lamp, is being employed.
Furthermore, here, a short arc type mercury discharge lamp means a mercury discharge lamp including a metal halide lamp and an ultrahigh-pressure mercury lamp, in which a coil that serves as the starting point of discharge at startup time is fixed on the lamp electrode so as to uniformly come into contact with the side surface of the electrode.
As for the startup of this kind of short arc type mercury discharge lamp, when the supply voltage is applied between the lamp electrodes, a glow discharge is started between one of the electrodes and a coil fixed to the side surface of the other electrode, and gradually shifts to an arc discharge. The heat of the coil heated by the discharge is transmitted to the electrode to which the coil is fixed, furthermore, this electrode is subjected to radiation heat due to the arc from the coil, which amounts to the state in which thermoelectrons are easily emitted. Furthermore, as the internal pressure of the lamp increases, the arc is narrowed down to be stable, and a stabilized arc discharge shifts to the tip of the electrode to generate a stationary lit up state of the lamp.
However, with the above-mentioned kind of short arc type mercury discharge lamp, the problem arises that the arc discharge commenced from the coil at startup time often becomes stabilized on the coil and does not shift to the tip of the electrode. This kind of phenomenon noticeably occurs when the tip of the coil is distanced from the tip of the electrode. This can be thought to occur because the heat produced due to the arc emitted by the coil, for example, escapes from the rear end part of the coil to the rear end part of the electrode which makes contact with the coil, and because thermal conduction to the tip of the electrode is not smoothly carried out, the occurrence of thermoelectron emission at the tip of the electrode becomes difficult and whereby the arc remains in the coil without being transferred to the tip of the electrode.
When this kind of abnormal discharge occurs, the arc contacts the outer wall of the lamp and troubles such as a lamp explosion, a cloudiness of the lamp, or a blackening of the lamp due to the vaporization of the tungsten coil due to abnormal heating of the coil occur. Furthermore, when glow discharge is continued for a long time, blackening of the lamp attendant upon the vaporization of the tungsten coil also occurs.
The present invention provides a short arc type mercury discharge lamp that, at lamp startup time, quickly shifts glow discharge to arc discharge, and quickly shifts the arc discharge position from the coil to the electrode tip, making it possible to maintain a stable discharge state. The short arc type mercury discharge lamp 1 is characterized in which a coil 5 has been arranged on the side surface of an electrode 4, and 0.15 mg/mm3 or more of mercury has been sealed inside a discharge vessel, wherein the side part 51 of the coil 5 close to the tip side of the electrode is constructed so as to be fixed to the side surface of the electrode 4, and, at least, the part 52 of the coil 5 following the side part 52 is distanced from the side surface of the electrode.
With the foregoing problems in view, an object of the present invention is to provide a short arc type mercury discharge lamp that, at lamp startup time, quickly shifts glow discharge to arc discharge, and quickly shifts the arc discharge position from the coil to the tip of the electrode, making it possible to sustain a stable discharge.
In the present invention, the following kinds of means are used in order to solve the above-mentioned problems.
The first means is characterized in that, a short arc type mercury discharge lamp having a coil placed on the side surface of on o of the electrodes, and 0.15 mg/mm3 or more of mercury hermetically enclosed in a discharge vessel, which is constituted so that the side part of the above-mentioned coil close to the tip side of the electrode is fixed to the side surface of the above-mentioned electrode, and, at least, the part of the coil following the side part of the coil is distanced from the side surface of the above-mentioned electrode.
The second means is characterized in that, in the above-mentioned first means, halogen is sealed in the discharge vessel.
The third means is characterized in that a short arc type mercury discharge lamp that has a coil arranged on the side surface of one of the electrodes, and, at least, emission metal and halogen sealed in a discharge vessel, which is constituted so that the side part of the above-mentioned coil is fixed to the side surface of the above-mentioned electrode, and, at least, the part of the coil following the side part of the coil is distanced from the side surface of the above-mentioned electrode.
The fourth means is characterized in that, in any one of the means, from the first means through the third means, the part of the coil close to the seal portion side of the above-mentioned electrode is constructed so as to be distanced from the side surface of the above-mentioned electrode.